linear foot?

First let me start out by saying I never claimed to be the sharpest pencil in the drawer, and I have a question that many of you will shake your head and think of me as stupid.
I have heard of linear foot, and sold by the linear foot, but rather than open my mouth and admit I am an idiot, I kept it shut and just let them think I was.
so now I admit it, what is a linear foot, and what kind of formula do you use to determine a linear foot.
thank you in advance.

Not to be confused with Bd. ft, or board feet of wood. . Anyway, to the original poster, there really is no such thing as a dumb question here. (Oh, certainly there are occasionally dumb answers, like some that I give, {the above is a fine example}). But, for the most part, there is ALWAYS something that each and everyone of us DOESN'T know.

Joe

Thank you, Dad - for always being the dad that you were. You truly are my hero. You always were.